The present invention relates to a mobile control device and handover control method, and more particularly to a mobile control device and handover control method in a mobile communication system that performs handover control of switching base stations that communicate with a mobile terminal as that mobile terminal moves.
FIG. 30 is a drawing showing the construction of a mobile communication system that uses IP (Internet Protocol), and comprises a mobile control device MCA, radio communication base stations BTS1 to BTSn and a plurality of mobile terminal MS (only one is shown in the drawing). The mobile control device MCA is connected to the IP network, which is a wired network, as well as is connected to the plurality of base stations BTS1 to BTSn using wired connections. The base stations BTS1 to BTSn perform radio communication with mobile terminals MS that are within cells CL1 to CLn.
Packets that are sent from a terminal (not shown in the figure) to a mobile terminal MS are sent to the mobile control device MCA that accommodates that mobile terminal. When the mobile terminal MS is located within the cell CL1, the mobile control device MCA sends the packets to the base station BTS1 that accommodates that cell, and then the base station BTS1 sends the packets to the mobile terminal MS using radio communication. After that, as long as the mobile terminal MS is located with the cell CL1, the mobile control device MCA performs control so that the communication between the sending terminal and the mobile terminal MS is performed using the path described above. However, when the mobile terminal MS moves and moves into an adjacent cell CL2, the mobile control device MCA performs handover control to switch the relay base station from the base station BTS1 to the base station BTS2.
In this mobile communication that uses IP, it is feasible to apply a Mobile IP (RFC2002, RFC3775) that makes mobility control in IP layers possible. By applying this technology to a mobile network, it becomes possible to continue IP communication even though as shown in FIG. 30, the base station with which the mobile terminal MS is connected changes during communication. Adopting Mobile IP as the mobility control protocol in next-generation cellular type mobile communication is being studied. In next-generation cellular type mobile communication a larger volume of high-speed communication is required than in the current mainstream 3G system, so in order to make high-speed packet transmission possible, a system is essential in which Mobile IP is performed at high speed.
Mobile IP gives a framework that is related to signaling necessary for performing packet transmission, however, detailed specifications related to packet transmission control are not given. Therefore, in order to perform Mobile IP at high speed, (1) technology for reducing the delay due to signaling, and (2) a transmission method for transmitting packets at high speed are necessary.
Hierarchical Mobile IPv6 Mobility Management (HMIPv6) (refer to Network Working Group Request for Comments 4140 (RFC4140) “Hierarchical Mobile IPv6 Mobility Management (HMIPv6)”) has been proposed as a method for accomplishing point (1) described above for performing Mobile IP at high speed. Moreover, Fast Handover for Mobile IPv6 (FHO) (refer to Network Working Group Request for Comments 4068 (RFC4068) “Fast Handovers for Mobile IPv6”), or a method of multicasting packets beforehand to a plurality of candidates as a target base station for communication with the mobile terminal (refer to Japanese patent application JP2004-282249A) have been proposed as methods related to point (2) described above. Furthermore, Hierarchical Mobile IPv6 with Buffering (HMIPv6-B) (refer to VTC-2003 Spring “Transmission Quality Evaluation for Hierarchical Mobile IPv6 with Buffering Using Test Bed”) that expands HMIPv6 and comprises a method of controlling packet transmission has been proposed. However, these methods have problems as described below.
Problems with HMIPv6:
HMIP is a method with the aim of reducing signaling in Mobile IP. HMIP is effective in increasing the speed of packet transmission by reducing signaling, however, it does not propose transmission technology itself for increasing the transmission speed.
Problems with FHO:
FHO proposes packet transmission technology for increasing the speed of packet. More specifically, FHO is a method of transferring in advance, remaining packets from the source base station to the target base station during handover of the mobile terminal. However, in this method, it is presumed that the target base station is already known, so there is a problem in that packets cannot be transferred when the target base station is not decided.
Problems with HMIPv6-B:
HMIPv6-B is a method that has a buffering function for buffering packets in addition to the other functions of HMIPv6, and at the same time takes into consideration increasing packet transmission. The buffering function is performed by a mobile control device that manages a plurality of handover base station candidates. With this method, it becomes possible to perform handover with reduced signaling delay and less packet loss. However, there is a problem in that delay time due to packet buffering increases.
Problems with Multicasting:
Multicasting is a method of multicasting packets beforehand to a plurality of base stations BTS2 to BTS4 that are candidates for handover of the mobile terminal MS during handover as shown in FIG. 31.
FIG. 32 is a drawing showing an example of construction of a mobile control device MCA and base stations BTS1 to BTSn in order to explain multicasting. The mobile control device MCA comprises: a wired network interface unit (hereafter, interface will be indicated as I/F) 101a that performs interface control between it and the IP network; a base station I/F unit 101b that performs interface control between it and the base stations; a transmission data processing unit 101c that performs communication control of user packets and control packets; and a handover/multicast control unit 101d that performs handover and multicast control. The base stations BTS1 to BTSn have the same construction, and comprise: a mobile control device I/F unit 102a that performs interface control between it and the mobile control device MCA; a radio communication I/F unit 102b that performs interface between control between it and the mobile terminal MS; a transmission data processing unit 102c that performs communication control of user packets and control packets; and a handover control unit 102d that performs handover control. The mobile control device MCA is connected to each of the base station BTS1 to BTSn by a single cable for each base station, and can freely transmit and receive control packets and user packets to or from the base stations by way of control channels and data channels.
FIG. 33 is a drawing showing the detailed construction of a mobile control device MCA, where the emphasis is placed on communication in the download direction. In the transmission data processing unit 101c, a packet copy unit 111 copies an input packet and inputs it to a transmission buffer 112 and a multicast unit 113. The transmission buffer 112 temporarily stores the input packet and transmits it to the suitable base station I/F unit 101b, and when the multicast unit 113 is instructed by the handover/multicast control unit 101d to perform a multicast, copies the input packet and multicasts that packet to the handover base station candidates BTS2 to BTS4 by way of the transmission buffers 114a to 114c and base station I/F 101b. In the handover/multicast control unit 101d, a handover control unit 121 performs overall handover control, and a multicast control unit 122 executes multicast control during handover.
FIG. 34 is a drawing showing the main parts of the handover control sequence when multicasting is employed, and FIG. 35 is a flowchart showing the flow of processing by the handover/multicast control unit 101d. 
The handover control unit 121 of the mobile control device MCA requests the mobile terminal MS to periodically measure and report the radio communication status while communicating with the base station BTS1. After receiving the request to measure and report the radio communication status, the mobile terminal MS measures the reception levels from all of the surrounding base stations BTS2 to BTSn and reports the results to the mobile control device MCA by way of the base station BTS1 that it is communicating with. After receiving this report, the handover control unit 121 makes reference to the reported signal levels and determines whether handover is necessary (steps 151 to 152), and when handover is necessary, decides a plurality of handover base stations whose signal levels exceed a threshold value as handover base station candidates (step 153), and notifies the multicast control unit 122 of those handover base station candidates. In FIG. 34, base stations BTS2, BTS3 and BTS4 are handover base station candidates.
After the handover base station candidates have been decided, the multicast control unit 122 checks whether the interface address, for example the MAC address (Media Access Control Address), of each of the candidate base stations is known, and when the MAC address is not known, sends a MAC address search packet to acquire the interface address of the base station in question. After this process is completed, the multicast control unit 122 sends a multicast instruction to the multicast unit 113. By doing so, the multicast unit 113 copies the packets and multicasts the packets to the handover base station candidates BTS2, BTS3 and BTS4 (step 154).
At the same time as the multicast described above, the handover control unit 121 determines a handover base station (step 155). For example, when the strength of the received electrical field from the base station BTS2 becomes a set value or greater, the handover control unit 121 decides the base station BTS2 as a handover base station (referred to as target base station). Next, the handover control unit 121 requests the target base station BTS2 to set a radio communication channel (step 156). Moreover, after receiving a radio communication channel setting response from the target base station BTS2 (step 157), the handover control unit 121 instructs the target base station BTS2 to send the multicast packets to the mobile terminal MS, and performs control so that packets received from the IP network are sent to the mobile terminal MS by way of the target base station BTS2. Furthermore, the multicast control unit 122 stops the multicast, and instructs the handover base station candidates that were not selected as the target handover base station to delete the multicast packets, and performs the process to end handover (step 158).
FIG. 36 is a drawing that explains the main parts of a different handover control sequence. In this example as well, the mobile terminal MS is communicating with the current base station BTS1, and base stations BTS2 to BTSn are located around the base station BTS1. By making reference to the reception levels of the surrounding base stations that are reported by the mobile terminal MS according to the same sequence as shown in FIG. 34, the mobile control device MCA decides handover base station candidates. In the example sequence shown in FIG. 34, the signal levels are reported by way of the base station (source base station) BTS1, however, in this example, the signal levels are reported by way of each of the base stations. In this case, when each of the base stations reports its signal level to the mobile control device MCA, it also notifies the mobile control device of its MAC address. Therefore, this sequence differs from the sequence shown in FIG. 34 in that the mobile control device MCA does not need to inquire of the interface addresses of the base stations, and by deciding the handover base station candidates the mobile control device MCA can perform multicasting immediately.
By using the multicasting method described above, the mobile terminal MS is able to quickly receive data from the handover base station BTS2 after handover is complete, so high-speed handover becomes possible. This technique can be applied to a mobile network that uses Mobile IP, so it is a powerful high-speed packet transmission method that is capable of improving the problems with the HMIPv6, FHO and HMIPv6-B methods.
However, in the conventional multicast method, the multicast transmission rate is decided according to the transmission rate of the source base station to which the mobile terminal MS is connected before handover. Therefore, when there are differences in communication capabilities among the plurality of handover base station candidates, for example, when the buffer sizes or radio communication access methods differ, a certain base station candidate is unable to store the multicast packets and thus the packets are deleted or rejected. As a result, a time delay occurs due to retransmission of the deleted packets, and thus there is a comprehensive possibility that high-speed handover will not be possible. FIG. 31 will be used to explain the deletion of packets in a conventional multicast in which the base station BTS1 was a source base station and packets are multicast to the handover base station candidates BTS2 to BTS3. The base stations BTS1 to BTS3 are base stations that conform to the IEEE802.11a standard (54 Mb/s Max.), and the base station BTS4 is a base station that conforms to the 3G standard (384 Kb/s Max.). In conventional multicast control, a single transmission rate of the multicast is determined based on the transmission rate of the base station BTS1, and the maximum transmission rate is 54 Mb/s.
The buffer capacity of each base station is designed so that the size corresponds to the transmission rate of the radio communication access method, so the buffer capacity of the handover base station candidate BTS4 is designed less than the buffer capacities of the handover base station candidates BTS1 to BTS3. Therefore, the transmission rate of 54 Mb/s of the packets that are multicast to the handover base station candidate BTS4 greatly exceeds the maximum transmission rate of 384 Kb/s that is allowed for a 3G base station, so it is impossible to store all the packets that are multicast in the buffer of the base station BTS4, and deleted since the buffer becomes full. In the case that the mobile terminal is handed over to the base station BTS4, the deleted packets must be transmitted again, and the delay time that occurs by retransmitting the packets makes it impossible to achieve high-speed handover.
This problem occurs not only in the case of handover between different types of systems as in the example described above, but also occurs in handover of identical systems. For example, this problem occurs in the source base station and target base station when there is a large difference with the signal level of the mobile terminal.
To sum up, during handover of the mobile terminal, when the mobile control device performs multicasting at a single transmission rate without taking into consideration the performance of the plurality of handover base station candidates, packets may be deleted in the plurality of handover base station candidates that are under the control of the mobile control device due to an overflow from the buffer, and the following problems occur.
First, when packets are deleted due to the overflow from the buffer, the deleted packets must be retransmitted at the end-to-end level, and thus there is a problem in that the transmission delay of the packets increases. Therefore, high-speed handover becomes impossible.
In addition, in the base station BTS4, due to the multicast packets, the resources that could be used by other mobile terminals are decreased, so there is a problem in that the performance of those terminals is hindered, as well as there is a problem in that it is not possible for a mobile terminal to begin new communication.